The operating environment created in advanced heat engines--like those to be found in the automobiles of tomorrow and aircraft capable of long distance, supersonic transfer--demands much from the components of such engines. To operate effectively under prolonged high stress--high temperature environments requires materials of high strength and reliability. Also, such materials may have complex shapes, and should be capable of mass production on an economically sound basis.
To satisfy such exacting operating and manufacturing criteria, efforts have been made to develop materials, like ceramics, which can operate reliably under stress at temperatures up to about 1500.degree. C. Such materials must have performance characteristics and properties which exceed those required to satisfy operating needs demanded by today's automobile, in which operating temperatures may approach about 800.degree. C.
To meet related design challenges, a need has arisen to develop high strength, high reliability ceramic materials, with the potential to form complex shapes which are suitable for components to be used in advanced heat engines. The fabrication method to be used should be adaptable to the mass production of complex parts on an economically sound basis.
Silicon carbide is one potential high temperature engineering material. Silicon carbide is one of the most available carbides, and its properties have long made it one of the most useful. Unlike many other carbides, however, silicon carbide is not easily sintered to produce a desirable result. This is probably explained in part by silicon carbide existing in many crystalline modifications which can be grouped in either an hexagonal or rhombohedral alpha SiC, or a cubic beta SiC, or a mixture of the alpha and beta forms. Structural complexity and heterogeneity results from the numerous stacking sequences which are possible in SiC crystals. For related reasons, silicon carbide has not easily been sintered to densities approaching those which are theoretically possible.
Another challenge offered by silicon carbides arises from transformation of the beta- to the alpha structure. During that transformation, large plate-like crystals of alpha SiC can occur in a matrix of fine grained beta SiC. Additionally, with alpha SiC material, discontinuous grain growth can be observed under unfavorable sintering conditions or if care is not taken to select dopants carefully.
As is now known, certain dopants or additives may have a significant effect on grain growth and consequent characteristics of the resulting material. Exaggerated, uncontrolled grain growth, for example, may produce anisotropy and have an undesirable effect on the mechanical properties of the resulting material.
Against a background of many experiments which have shown that pure SiC cannot sinter, even beginning with a sub-micron fine powder under normal sintering conditions, the quest for identifying and selecting appropriate additives has been pursued for some time.
The use as engine components of ceramic materials such as silicon carbide has been reported in JA-151708 which was published on Dec. 6, 1980. That reference discloses a valve guide for an internal combustion engine wherein a primary design criterion is wear-resistance, the valve guide consisting of silicon carbide or nitride.
U.S. Pat. No. 4,881,500 which issued on Nov. 21, 1989 discloses a poppet valve made of ceramic such as silicon nitride or sialon. That reference describes valve operating conditions which are different from those contemplated by the present invention. In the valve of that reference, there generally is a relatively low thermal conductivity. Therefore, areas of localized heat intensity may develop which require alleviation by reshaping the valve surface.
U.S. Pat. No. 4,928,645 which issued on May 29, 1990 discloses a ceramic composite valve for internal combustion engines. That reference discloses ceramic sleeving of strands and fibers which may result in anisotropic properties and probable failure.